Crystalline electric field effects in Ce M In 5 ( M = Co , Rh , Ir ) : Superconductivity and the influence of Kondo spin fluctuations

We have measured the crystalline electric field (CEF) excitations of the $\mathrm{Ce}M{\mathrm{In}}_{5}\phantom{\rule{0.3em}{0ex}}(M=\mathrm{Co},\mathrm{Rh},\mathrm{Ir})$ series of heavy fermion superconductors by means of inelastic neutron scattering. In each case, the CEF excitations are considerably broadened, due to Kondo hybridization of the localized $f$-moments with the conduction electrons. Fits to a phenomenological CEF model reproduce the inelastic neutron scattering spectra and the high-temperature magnetic susceptibility. We also present calculations within the noncrossing approximation (NCA) to the Anderson impurity model, including the effect of CEF level-splitting for the inelastic neutron scattering spectra and the magnetic susceptibility. Our results indicate that the CEF level-splitting in all three materials is similar, and can be thought of as being derived from the cubic parent compound $\mathrm{Ce}{\mathrm{In}}_{3}$ in which an excited state quartet at $\ensuremath{\sim}12\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ is split into two doublets by the lower symmetry of the tetragonal environment of the $\mathrm{Ce}M{\mathrm{In}}_{5}$ materials. The evolution of the superconducting transition temperatures in the different members of $\mathrm{Ce}M{\mathrm{In}}_{5}$ can be understood as a direct consequence of the strength of the $4f$\char21{}conduction electron hybridization.